In nuclear medicine, images of internal structures or functions of the body are acquired by using gamma cameras to detect radiation emitted by a radio-pharmaceutical that has been injected into the patient's body. A computer system controls the gamma camera to acquire data and then processes the acquired data to generate the images. Nuclear medicine imaging techniques include single-photon emission computed tomography (SPECT) and positron emission tomography (PET). SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma ray pairs emitted in coincidence in opposite directions due to electron-positron annihilations. Accordingly, PET imaging is sometimes referred to as coincidence imaging.
One problem encountered in nuclear medicine is associated with transmission scanning for purposes of generating an attenuation map. Gamma ray detectors are typically collimated during SPECT imaging. A conventional parallel hole lead collimator is often used. The associated transmission scan involves the use of a radiation source to transmit gamma rays through the patient to a collimated detector. For example, in some prior art systems, a line source is scanned directly across the field of view (FOV) of the corresponding detector. The configuration and positioning of the line sources allows a substantial number of the transmitted photons to pass through the holes of the collimator to the detector, without impinging on the septa of the collimator. Such photons are not absorbed by the collimator, and there is therefore no need to remove the collimator for purposes of performing the transmission scan.
For various reasons, however, it may be desirable to position a transmission source in a gamma camera system such that the paths of the transmitted photons will not line up with the holes of the collimator, in contrast with the above-mentioned system. For example, it may be desirable to position the source outside the FOV of the corresponding detector. As a result, if a conventional SPECT collimator and singles transmission source is used, few, if any, of the transmitted photons will pass through the collimator to the detector, due to the incident angle of the photons relative to the holes of the collimator. Therefore, in such a system it might be necessary to remove the collimator before performing the transmission scan and to replace it with a collimator having a different physical configuration. Because collimators tend to be quite large and heavy, removal and changing of collimators can be time-consuming and difficult. In a clinical setting, the amount of time spent conducting an imaging session may impact the patient's comfort and may be critical to the patient's health. It would be undesirable, therefore, to increase the overall time required to complete an imaging session by requiring the changing or removal of collimators. Hence, what is needed is a technique for performing transmission scanning in a nuclear medicine imaging system which overcomes this problem.